Pest organisms have a detrimental impact on the areas they infest, and commonly cause economic, environmental and/or social problems. As a consequence there is ongoing demand for safe, humane and effective methods for their management and control.
Organisms ranging from invertebrate herbivores such as snails and caterpillars to vertebrate predators such as foxes, coyotes and feral cats are considered to be pests in different parts of the world. A number of amphibians are also considered to be pest organisms; these include species such as the cane toad (Bufo marinus), the Cuban tree frog (Osteopilus septentrionalis), the American bullfrog (Lithobates catesbeiana) and the coqui frog (Eleutherodactylus coqui). Each of these amphibian species has significant impacts in areas they infest, with effects that include displacement of native amphibia, poisoning of native wildlife and domestic animals, and loss of social amenity.
The cane toad (Bufo marinus) is a significant pest in Australia, continental USA, and various other Pacific and Caribbean locations. The cane toad was deliberately introduced into Australia in 1935 in an attempt to control beetle pests of sugarcane in northern Queensland (Lever, C., 2001). The toad established effectively at points of release, but proved completely ineffective with respect to its intended role. Since then the cane toad has spread widely across the Australian continent, such that its distribution now extends from northern New South Wales, through coastal and northern regions of Queensland, and into the western parts of the Northern Territory. Isolated populations are also found in various urban centres such as Katherine, where toads apparently rely on the suitable habitat associated with water in parks, gardens and recreational areas. It is expected that the distribution of the toad will continue to extend westwards across northern Australia, to ultimately cover the entire “Top End” of the continent, including the Kimberley and the Ord River regions. Some authorities have also suggested that the toad has the potential to colonise the warmer northern regions of the Murray-Darling basin.
Establishment of the cane toad is considered to have significantly impacted the Australian environment. In this regard, the cane toad's capabilities as a predator of native invertebrates and a competitor of native amphibia, together with its capacity to poison reptiles and other animals by which it is mouthed or ingested, are believed to have significantly affected endemic species in freshwater and riparian settings (Doody, J. S. et al., 2006). The biotic impact of cane toads has recently been listed under the Commonwealth Environment Protection and Biodiversity Act (EPBC Act) as a “key threatening process” to the Australian environment.
Cane toads were also deliberately released in Florida, also in attempts to control insect pests of sugarcane. In addition, an accidental release at Miami International Airport appears to have been important for their establishment in the state (Youth, H., 2005). In the USA, as in Australia, the cane toad consumes, competes with, and poisons native wildlife.
In both Australia and the USA, the cane toad also has a detrimental impact on the “quality of life” in urban settings, where its presence in areas such as domestic yards and parkland areas poses a threat of poisoning to domestic pets (Roberts, B. K. et al., 2000; Reeves, M. P., 2004), and a risk that children may be exposed to the toxin of the pest.
Another significant amphibian pest in south-eastern USA is the Cuban tree frog (Osteopilus septentrionalis). The frog is thought to have been introduced into Florida from its area of endemicity in the Caribbean by accidental transport in shipping crates (Johnson, S. A. 2007). The species has now expanded its range to include much of Florida, and individuals have also been found in the state of Georgia (Knapp, W. W., 2007).
As with cane toads, Cuban tree frogs have “negative impacts on Florida's native species and ecosystems” (Johnson, S. A., 2007), principally because of their competitive capabilities and predatory behaviour. Cuban tree frogs also secrete a material that coats their skin and that is “extremely irritating” to the eyes and nose of persons (Johnson, S. A., 2007). Cuban tree frogs also impact on social amenity through their propensity to shelter in houses, and in other structures such as electrical transformer boxes, from where they can cause short-circuits and resultant power outages to the surrounding area (Johnson, S. A., 2007).
Two other invasive amphibian species are also worthy of note, having, like the cane toad, been nominated by the Invasive Species Specialist Group (ISSG) as members of the “World's Worst Invasive Alien Species” (International Union for the Conservation of Nature ISSG, 2006). These species are Eleutherodactylus coqui (the “coqui”, a small tree frog native to Puerto Rico) and Rana catesbeiana, the North American bullfrog.
Humane ways of killing pest amphibia are not available to most householders, either because they involve the need for restricted drugs or poisons or a degree of specialist training. Gassing and freezing are commonly used methods for killing amphibia; each, however, requires physical handling and the transportation of live animals, which can pose a risk to handlers. Further, these methods can be difficult to conduct in remote settings. While freezing is the recommended method for destroying cane toads in domestic settings in some jurisdictions, most householders find freezing undesirable due to the possibility of contamination of food with the toxin. As a consequence, a variety of broadly unacceptable “techniques”, commonly involving physical violence, are adopted, particularly for cane toad control. Outcomes of these methods can be uncertain, and the use of violent means of control is considered to be socially unacceptable by a large segment of the population. In addition, there are reports of persons being adversely affected by toad toxins in the course of attempting to kill the animals (Wilson, A., 2006). This can be explained, in many cases, by discharge of the toxin following dubbing of the animal, and subsequent contact with and poisoning through the skin or mucosal surfaces.
To overcome these problems there is growing interest in the use of various forms of traps to catch pest amphibia, such as cane toads (Sawyer, G., 2006). While some of these can apparently operate effectively, captured amphibia must still be killed by some means.
Against this background, it can be appreciated that a method of killing pest amphibians such as cane toads and Cuban tree frogs in an acceptable timeframe, while providing a humane mode of action and convenience of use, would provide a valuable advance in efforts to manage the pests.
The present applicant proposes herein a pesticidal composition for use on pest amphibians that is relatively safe to use, that may be suitable for household use and which, in addition, removes the need for specialist equipment and/or training in order to apply. Further, in the contexts of cane toad and Cuban tree frog control, the proposed pesticidal composition removes the risk of harm from exposure to the cane toad toxin to those who would normally use physical means to destroy the animals.